As a conventional oil passage change-over valve, there has been used a three way valve device disclosed in, for example, Japanese Patent 3122224. In this three way valve device, a first opening portion is formed in one side of a valve body and a second opening portion and a third opening portion are provided in another side thereof, a valve element is rotatably provided inside a valve hole of the valve body, and a straight communicating oil passage is formed in this valve element for alternately communicating the first opening portion with one of the second opening portion and the third opening portion. Inside this valve body, there are formed a first oil passage for connecting the valve hole with the first opening portion, a second oil passage for connecting the valve hole with the second opening portion and a third oil passage for connecting the valve hole with the third opening portion.
In this three way valve device, the second oil passage and the third oil passage are separated by a thin partition and both of the passages are formed to be straight passages with their axial centers being in parallel and adjacent to those of the second and third opening portions.